The present invention relates to a driver circuit, an electro-optical device, and an electronic instrument.
As a liquid crystal panel (electro-optical device) used for an electronic instrument such as a portable telephone, a simple matrix type liquid crystal panel and an active matrix type liquid crystal panel using a switching device such as a thin film transistor (hereinafter abbreviated as “TFT”) are known.
The simple matrix type liquid crystal panel allows power consumption to be easily reduced in comparison with the active matrix type liquid crystal panel. However, the simple matrix type liquid crystal panel has disadvantages in that it is difficult to increase the number of colors and to display a video image. The active matrix type liquid crystal panel is suitable for increasing the number of colors and displaying a video image. However, the active matrix type liquid crystal panel has a disadvantage in that it is difficult to reduce power consumption.
In recent years, a video image display of an increased number of colors has been increasingly demanded for a portable electronic instrument such as a portable telephone in order to provide a high-quality image. Therefore, the active matrix type liquid crystal panel has been increasingly used instead of the simple matrix type liquid crystal panel.
In the active matrix type liquid crystal panel, it is desirable to provide an operational amplifier functioning as an output buffer in a data line driver circuit which drives data lines of the liquid crystal panel.
FIG. 21 shows a configuration of a known operational amplifier.
This operational amplifier is disclosed in JP-A-2003-157054. In this operational amplifier, an n-type driver transistor M10 is controlled by a p-type differential input circuit including p-type transistors M7 and M8, n-type transistors M5 and M6, and a current source CSb. A p-type driver transistor M9 is controlled by an n-type differential input circuit including p-type transistors M1 and M2, n-type transistors M3 and M4, and a current source CSa.
Consider the case where the voltage of an input signal Vin is higher than the voltage of an output signal Vout for the n-type differential input circuit. In this case, since the impedance of the n-type transistor M4 becomes higher than the impedance of the n-type transistor M3, the gate voltage of the p-type transistors M2 and M1 increases, whereby the impedance of the p-type transistor M1 increases. Therefore, the gate voltage of the p-type driver transistor M9 decreases, whereby the p-type driver transistor M9 approaches the ON state.
In the p-type differential input circuit, when the voltage of the input signal Vin is higher than the voltage of the output signal Vout, since the impedance of the p-type transistor M8 becomes smaller than the impedance of the p-type transistor M7, the gate voltage of the n-type transistors M5 and M6 increases, whereby the impedance of the n-type transistor M5 decreases. Therefore, the gate voltage of the n-type driver transistor M10 decreases, whereby the n-type driver transistor M10 approaches the OFF state.
As described above, when the voltage of the input signal Vin is higher than the voltage of the output signal Vout, the p-type driver transistor M9 and the n-type driver transistor M10 operate in such a manner that the voltage of the output signal Vout increases. An operation reverse of the above-described operation is performed when the voltage of the input signal Vin is lower than the voltage of the output signal Vout. As a result of the above-described operation, the operational amplifier transitions to an equilibrium in which the voltage of the input signal Vin is approximately equal to the voltage of the output signal Vout.
However, the input signal Vin is supplied to the p-type transistor M7 as the gate voltage in the p-type differential input circuit, and the input signal Vin is supplied to the n-type transistor M3 as the gate voltage in the n-type differential input circuit. Therefore, as shown in FIG. 22, an input dead zone in which the voltage of the input signal Vin and the voltage of the output signal Vout cannot be made equal occurs in a range R1 in which the input signal Vin is set at a high-potential-side power supply voltage VDD to “VDD−|Vthp|” (Vthp is the threshold voltage of the p-type transistor M7) and in a range R2 in which the input signal Vin is set at a low-potential-side power supply voltage VSS to “VSS+Vthn” (Vthn is the threshold voltage of the n-type transistor M3). This is because the n-type differential input circuit does not operate in the range R2 between the low-potential-side power supply voltage VSS and “VSS+Vthn” since the n-type transistor M3 remains in the OFF state, and the p-type differential input circuit does not operate in the range R1 between the high-potential-side power supply voltage VDD and “VDD−|Vthp|” since the p-type transistor M7 remains in the OFF state.
For example, consider the case of driving a liquid crystal panel at 64 grayscales using a grayscale voltage having a maximum amplitude of 5 V (VinR). In this case, if the amplitude of 5 V is reduced in order to generate a grayscale voltage corresponding to each grayscale, the grayscale representation is impaired. Therefore, an offset of about 1.9 V is provided taking into consideration the variations of the threshold voltage Vthp of the p-type transistor and the threshold voltage Vthn of the n-type transistor to generate a grayscale voltage having a maximum amplitude of about 6.9 V (VDDR). As a result, when the power supply system of the data line driver circuit is 5 V, it is necessary to provide a voltage booster circuit in order to generate a grayscale voltage having an amplitude of about 6.9 V. When using a charge-pump circuit as the voltage booster circuit, transistors and capacitors for increasing the voltage are additionally required, and a layout taking a high voltage into consideration becomes necessary. Therefore, the chip area, total mounting cost, and power consumption are increased. In particular, since a 5-volt process for a logic power supply is insufficient, it is necessary to use a high-voltage transistor of 7 V or more, whereby the manufacturing cost is increased.
In the operational amplifier having a configuration shown in FIG. 21, the p-type driver transistor M9 and the n-type driver transistor M10 cannot be controlled when the input signal Vin in the input dead zone is input, whereby a shoot-through current cannot be prevented. This causes a decrease in circuit stability and an increase in power consumption.
Moreover, the operational amplifier constantly consumes an operating current. Therefore, even if a circuit configuration which prevents the above-described input dead zone is employed, a reduction in power consumption may not be achieved due to an increase in the number of current paths and the like.